Pure paranoia.
You are still missing my point, IF this is a PC based product then it is designed to run one thing - a pinball machine - that's it, nothing else, NADA, ZIP, diddly squat.
Therefore maintenance and product life comes into it. We are still maintaining pinball machines now with the boards you and others produce, coil sleeves, coils etc. the list goes on.
Absolutely NOT! This is my entire point. I'm not getting personal - it's FACT....
If you fit one of my boards, you screw it into the machine, plug it in and away it goes - JOB FINISHED.....to replace a motherboard you need to install all new SOFTWARE drivers (that suit the new board) and probably replace RAM, micro, power supply etc etc -
This is VASTLY different to repairing an embedded pinball and will likely be beyond most people.
Where the problem lies is the complexity of a PC - it is made up of a mainboard or motherboard, CPU, RAM, power supply, and some sort of disk. All of these are designed to work together and there is probably an 18-24 month window, sometimes more but sometimes less, before stocks of certain items become unavailable. Intel seem to have numerous CPU form factors which quickly become obsolete - AMD seems to be a lot better in that respect. If a component dies in 4 years time, it is highly likely that you will not be able to easily replace that component unless it is the disk (however, after the disk, the motherboard is most likely to go). You will then be forced to replace the motherboard and CPU, and most likely RAM as the old RAM is unlikely to work in the new motherboard. You are also likely to have to replace the power supply because it won't have the new connectors required.
OK, so you have the computer physically working again. But will the operating system still work on the new computer? Again, unless the manufacturer has kept the operating system up to date and working on current PC technology, this is highly unlikely. Depending on what was used as a base (eg Linux) you may get lucky and be able to update the OS to the point where it does work. Or perhaps some genius can write drivers for you if they are using some other OS. This is what happened to Pin 2000 - it relies on a specific CPU/GPU component (the Cyrix MediaGX CPU) that is no longer available BUT the software is written around it and won't work.
OK, now you have the computer and software working. But what about the interface from the hardware to the PC? Unless the manufacturer has used something generic (such as USB inputs which will still be around) then it is probable they have developed an interface card using PCI or PCI Express. But in 4 years time PCI will be totally gone - PCI Express will still be around and SHOULD be backwards compatible. Fast forward a few more years and PCI Express is a dinosaur.
Whether the PC is running one application or not is totally irrelevant to the lifespan of the PC (although it should stop it from crashing too much due to software issues).